The present invention generally relates to flip chip semiconductor devices. More particularly, this invention relates to a method of conducting and dissipating heat from flip chips for purposes of thermal management of such devices, and to a heat-conducting structure for this purpose.
A variety of methods are known for dissipating heat generated by semiconductor devices. Such methods typically entail the use of a thermally-conductive substrate, such as beryllium oxide (BeO), or a substrate modified to promote its heat conduction capability. Significant disadvantages with the use of BeO substrates include the relatively high cost of manufacture and the toxic nature of BeO. In the case of a substrate modified to promote its thermal conductivity, thermal management is usually achieved by dissipating heat primarily in the vertical, or xe2x80x9czzxe2x80x9d direction, beneath the semiconductor device. For example, heat-generating semiconductor chips, such as power flip chips, are often mounted to alumina substrates that conduct and dissipate heat in the vertical direction away from the chip. These designs are limited in their ability to dissipate heat laterally, i.e., in the xe2x80x9cxxe2x80x9d and xe2x80x9cyxe2x80x9d directions, because the thermal conductivity of alumina is low compared to metals and metal-containing materials.
Another approach to dissipating heat from power semiconductor devices entails the use of ultra-thick films (UTTF), as taught in U.S. Pat. No. 5,527,627 to Lautzenhiser et al. Whereas conventional thick films are generally limited to thickness of up to about 0.001 inch (about 25 micrometers), Lautzenhiser et al. teach an ultra-thick film conductor having a thickness of about 0.005 inch (about 125 micrometers) or more, and therefore more capable of conducting heat laterally, in the x and y directions, from a heat-generating semiconductor device. However, the use of ultra-thick thick-film conductors is not always practical, as is the case when the heat-generating device is a flip chip, and particularly power flip chips that operate at power levels of 5 watts and higher.
It is an object of this invention to provide a method for conducting heat from a flip chip semiconductor device mounted to a substrate.
It is another object of this invention that the method entails a structure that thermally contacts a surface of the flip chip, such that heat generated by the chip is conducted away from the chip and the substrate to which the chip is mounted.
In accordance with a preferred embodiment of this invention, these and other objects and advantages are accomplished as follows.
According to the present invention, there is provided a method and assembly for conducting heat from a flip chip semiconductor device, and particularly a power flip chip. The assembly is generally constructed to dissipate heat from a flip chip mounted to a flexible substrate, such as a laminate or a printed wiring board. The flip chip has solder bumps on a first surface registered with conductors on the substrate, and a second surface oppositely disposed from the first surface. Heat is conducted from the flip chip with a heat-conductive member brought into thermal contact with the second surface of the flip chip. The substrate is biased against the flip chip so as to maintain thermal contact between the flip chip and the heat-conductive member. A thermally-conductive lubricant is preferably provided between the second surface of the flip chip and the heat-conductive member. The lubricant serves to fill gaps between the flip chip and heat-conductive member in order to promote thermal contact, while also serving to decouple any lateral mechanical strains that may arise as a result of different thermal expansions and movement between the flip chip, substrate and heat-conductive member.
In a preferred embodiment, the substrate and flip chip are enclosed within a housing that incorporates the heat-conductive member, which may be a pedestal projecting into the interior of the housing. With such a structure, a biasing member such as a spring or elastomeric member can also be enclosed in the housing, such that the housing supports the biasing member to maintain the biasing member in contact with the substrate, and therefore the flip chip in contact with the heat-conductive member. The housing may further include convection cooling fins in order to promote heat dissipation from the housing to the surrounding environment.
From the above, one can see that the heat-conducting assembly of this invention provides an uncomplicated method for dissipating heat from a flip chip. Instead of relying on heat conduction through the substrate via the flip chip solder bumps, as conventionally done, the present invention conducts heat away from the chip through the back side of the chip, i.e., the surface of the chip opposite the surface on which the flip chip microcircuitry is formed. The heat-conductive member serves both as a conductive path from the flip chip and a heat sink, the latter being promoted if the heat-conductive member is in the form of a pedestal, and particularly a pedestal formed as an interior projection of a housing.
Other objects and advantages of this invention will be better appreciated from the following detailed description.